The present invention relates generally to communications systems and more particularly relates to a half duplex digital communications system that utilizes a single filter for both transmit and receive purposes.
Digital communications systems are currently in wide spread use. The number of products incorporating communications features (wireless or wired) is growing at an ever increasing rate. A factor fueling the decision to add communication features to products that were traditionally without communications capability is the decreasing cost and the reduced space required by the communications circuitry. Large cost reductions combined with smaller footprints have made it practical and economic for manufacturers to add communications features to their products.
Manufacturers and designers are constantly attempting to reduce the cost and size of communications circuitry. Thus, it is desirable to be able to reduce the size and cost of communications systems even further.
A block diagram illustrating a prior art half-duplex communications transceiver is shown in FIG. 1. The communications transceiver, generally referenced 10, comprises a host interface 12 and a control module 28. The host interface is coupled to a host computer, processor, circuit or other computing device and functions to exchange transmit, receive and control related data to and from the transceiver 10. The transmit path comprises transmitter 14, Tx filter 16 and output Tx buffer 18. The receive path comprises input Rx buffer 22, Rx filter 24 and receiver 26. A communication medium interface 20 couples the Tx buffer 18 and the Rx buffer 22 to the communications medium.
The transceiver 10 functions as a bidirectional half duplex transceiver node. The control module 28 functions to enable and control the receiver 26, Rx buffer 24, transmitter 14 and the Tx buffer 18. In operation, the host supplies data to be transmitted to the transmitter 14. The transmitter 14 functions to encode and process the data. The signal output of the transmitter passes through a Tx filter 16 to filter out unwanted signal components. The filtered signal output of the Tx filter 16 is then amplified via output amplifier/buffer 18 and subsequently coupled to the transmission medium via communications medium interface 20.
During reception, the signal is received from the communications medium via the communications medium interface 20. The receive signal is buffered via Rx input buffer 24 and subsequently input to a Rx filter 24 which functions to remove any out of band noise and interference present on the medium. The filtered receive signal is then input to the receiver 26 where it is processed and decoded. The receiver then outputs the signal to the host interface 12 which sends it to the host.
Note that the prior art communications transceiver node described above is general in nature with only basic components shown and described. Each transceiver node in the communication network, however, comprises these basic elements and may have additional filters, buffers, amplifiers, etc. as part of its circuitry.
A disadvantage of the prior art transceiver node described in FIG. 1, is that depending on the parameters of the components, the use of both Tx and Rx filters in the transceiver may be superfluous. Since the transceiver is half duplex, only one filter is used at a time. Thus, the component count can be reduced if one filter were shared for both transmit and receive functions. This would reduce the size and cost of the communications transceiver.
The present invention is a communications transceiver constructed to minimize the additional external components required for interfacing to the communications medium. The principles of the present invention are illustrated using as an example, a half duplex receiver/transmitter that embodies a reduced component count design. The invention also includes a communications transceiver with a reduced number of filters.
The communications transceiver comprises a host interface, a control module, a transmit portion and a receive portion. The host interface is coupled to a host computer and functions to exchange transmit, receive and control related data to and from the transceiver. The transmit portion comprises a transmitter, analog switch, dual function filter and output Tx buffer. The receive portion comprises an input Rx buffer, dual function filter and receiver. A communication medium interface couples the Tx output buffer and the Rx input buffer to the communications medium.
In accordance with a key feature of the invention, the same filter is used for both receiving and transmitting. The single filter plays a dual role, providing filtering functions during both receive operation and transmit operation. This serves to minimize the external components required, the cost of the device and its complexity.
The filter functions to remove out of band noise and interference present on the medium. The filtered receive signal is input to the receiver where it is processed and decoded. The receiver then outputs the signal to the host interface which forwards the receive data it to the host.
A switching device, such as an analog switch, performs the switching function of connecting the filter either to the output of the transmitter during transmission or connecting the filter to the output of the communications medium interface during reception. During transmission, an output amplifier functions to amplify the output of the filter before the communication medium interface couples the signal onto the medium. During reception, the output amplifier is disabled and the signal received from the communications medium interface is filtered and then input to the receiver.
There is provided in accordance with the present invention a communications transceiver coupled to a host and to a communications medium comprising a host interface coupled to the host, the host interface operative to transmit and receive data and control information to and from the host, a communications medium interface coupled to the communications medium adapted to electrically couple the transceiver to the communications medium, a transmitter coupled to the host interface, the transmitter adapted to encode and modulate a transmit signal in accordance with transmit data to be transmitted, a receiver coupled to the host interface, the receiver adapted to decode and demodulate signals received from the communications medium interface so as to yield receive data, a filter coupled to the receiver and to the communications medium interface, the filter adapted to filter signals received from the communications medium interface and to filter signals output of the transmitter and a switch including a first input, a second input and an output, the first input coupled to the output of the transmitter, the second input coupled to the communications medium interface, the output coupled to the filter, wherein in a transmit mode the switch is operative to couple the first input to the filter, and wherein in a receive mode, the switch is operative to coupled the second input to the filter.
The switch further comprises switching time reduction means for reducing the transient response at the output of the filter, and wherein the switching time reduction means comprises an RC network.
There is also provided in accordance with the present invention, in a communications transceiver coupled to a host and to a communications medium, a method of processing receive and transmit signals, the method comprising the steps of providing a host interface for transmitting and receiving data and control information to and from the host, providing a communications medium interface for interfacing to the communication medium, encoding and modulating a transmit signal in accordance with transmit data to be transmitted, decoding and demodulating signals received from the communications medium so as to generate receive data, providing a single filter for filtering signals received from the communications medium interface and for filtering signals to be transmitted over the communications medium, switching, during a transmit mode of operation, the signal input to the single filter to be the signal to be transmitted onto the communication medium and switching, during a receive mode of operation, the signal input to the single filter to be the signal received from the communication medium.